Thickeners are used in numerous products for Theological control purposes and particularly for increasing viscosity and imparting thixotropic properties to the products. Thickeners for water dispersed compositions require compatibility and stability, especially in latex paints. Paint coatings are surface coatings applied to substrates and dried to form continuous films for decorative purposes as well as to protect the substrate. Consumer paint coatings are air-drying and primarily decorative architectural coatings applied to interior or exterior surfaces, where the coatings are sufficiently fluid to flow out and form a continuous paint film, and then dry at ambient temperatures. Industrial maintenance coatings are similar coatings applied to substrates in industrial environments to primarily protect the substrate.
A paint coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives. In dried paint films, the polymeric binder functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness of the dried paint film, although some paint coatings contain little or no opacifying pigments and can be described as clear or opaque coatings. The manufacture of paint coatings involves the preparation of polymeric binder, mixing of component materials, grinding of pigments in a dispersant medium, the addition of thickeners for rheology control, and thinning to commercial standards.
Latex paints for consumer and industrial maintenance markets typically are based on polymeric binders prepared by emulsion polymerization of ethylenic monomers. A typical latex paint binder, for instance, contains vinyl acetate copolymer consisting of major amounts of polymerized vinyl acetate copolymerized with lesser amounts of acrylate monomer such as butyl acrylate. Other polymeric binders are primarily or entirely acrylic copolymer latex emulsions. The hardness of the latex polymer must be balanced to permit drying and film formation at ambient application temperatures, which requires soft polymer units, while at the same time being hard enough to provide dried film resistance properties, which requires hard polymer units. This is conventionally accomplished by designing a latex polymer with a moderately elevated Tg (glass transition temperature) but then lowering the Tg temporarily with a volatile organic coalescing solvent. Coalescing solvents function to temporarily plasticize the latex polymer for time sufficient for the polymer particles to coalesce and develop film formation.
Due to the continuous aqueous phase in latex polymers, latex paints must contain dispersants and thickeners to promote adequate suspension of the pigment along with proper application rheology and flow out. The paint viscosity during storage must be adequately high to prevent settling, but readily reduced by applied shear to spread and flow out evenly. Latex paint typically exhibits thixotropic rheology to enable the paint to be applied readily by brush or roller or spray application. On a vertical wall, thixotropy will enable the applied paint to flow into a smooth continuous paint film without sagging.
For many years, the thickeners of choice for latex paints were derivatives of cellulose, including carboxymethyl cellulose and hydroxy ethyl cellulose. Other thickeners commonly used are polyethylene oxide copolymers commonly known as "Tween" or "Carbowax" thickeners, but these thickeners cause water sensitivity to the dried paint film due to the polyethlene oxide molecular structure. In recent years, the polyoxide thickeners have been largely replaced by associative thickeners, including hydrophobically modified hydroxyethyl cellulose and hydrophobically modified polyethylene glycols with the terminal hydrophobes attached by urethane linkages (HEUR). Other thickeners are acrylic polymers modified with inert non-reactive hydrocarbyl terminated compound such as shown in U.S. Pat. No. 5,292,828, EP 705,852, WO 9324544, and U.S. Pat. No. 5,405,900 with hydrophobic modification. All of these thickeners provide acceptable application properties, including roller spatter resistance and good flow and leveling, but are high cost materials and cause considerable water sensitivity in the dry paint film.
It now has been found that high molecular weight polymeric thickeners for water dispersed products generally, and especially useful for thickening aqueous emulsion latex paints, can be produced directly in water without utilizing volatile organic solvent or hydrocarbon diluents by copolymerizing an aqueous miniemulsion of ethylenic monomers in water to directly produce a miniemulsion polymeric thickener. In latex paints, the miniemulsion copolymer functions as a thickener for rheological control purposes as well as an effective dispersant for pigment dispersion into the latex polymeric binder. In accordance with this invention, the high molecular weight thickener is produced by first mixing together a mixture of specific ethylenic unsaturated monomers including acrylic or methacrylic acid monomer, a long chain alkyl methacrylate or acrylate, along with a methyl or ethyl ester of acrylic or methacrylic acid. The mixture of monomers is first blended together, mixed with surfactant and water, and then microdispersed into water under high shear to produce a water dispersed miniemulsion of ethylenic monomers. The preformed miniemulsion of organic monomers is then copolymerized to produce an aqueous miniemulsion copolymer having a particle size below 5 microns and preferably between 0.1 and 1.0 microns. Forming an aqueous miniemulsion of monomers prior to copolymerization enables formation of high molecular weight polymeric thickener in water directly and without the assistance of organic solvents. The preformed miniemulsion insures uniform monomer composition during aqueous polymerization and avoids the partitioning of the long chain alkyl acrylate and methcarylate ester monomers into large droplets.
In conventional latex polymerization, monomer added directly to a mixture of surfactant and water ordinarily forms monomer droplets usually several hundred microns in diameter. Monomers then diffuse into the aqueous phase and polymerize in tiny polymer particles, which are less than 1 micron in diameter. However, conventional latex polymerization will not work with the mixture of monomers described above, because the long chain alkyl acrylate and methacrylate esters are not water soluble enough to diffuse from the large monomer droplets into the polymer particles. Ordinarily the long chain alkyl acrylate and methacrylate ester monomers become separated and fail to copoplymerize with carboxyl monomers and lower alkyl ester and similar monomers. Consequently, the long chain alkyl ester monomers remain unreacted in the large monomer droplets, while the other monomers copolymerize without them in the polymer particles. Miniemulsion polymerization in accordance with this invention circumvents this problem by pre-emulsifying the monomer mixture (including the long chain alkyl ester monomers) to a sub-micron particle size. These preformed miniemulsions are stabilized by the water insoluable long chain alkyl ester monomers. Polymerizaton then takes place in these uniform monomer sub-micron droplets to give a uniform polymer composition which includes copolymerized long chain alkyl ester monomers.
Acrylic polymeric surfactants and thickeners in organic solvent have been disclosed in several patents. For example, U.S. Pat. No. 4,509,949 teaches polymeric thickeners with compositions of about 96% acrylic acid, 3% stearyl methacrylate, and 1% allyl crosslinker. However, the solvent must be removed from the polymer prior to use in a process that is difficult and expensive. Similarly, U.S. Pat. No. 5,288,814 suggests comparable acrylic thickeners prepared in solvent with a steric stabilizer to aid in processing. U.S. Pat. No. 5,057,241 and 4,351,754 suggest an acrylic thickener that has a composition of about 40% methacrylic acid, 50% ethyl acrylate, and 10% of a acrylate or methacrylate monomer, where the monomer contains a polyethylene glycol block of about 10 to 20 units and a terminal alkyl group of about 15 to 20 carbons. The polyethylene oxide structure however causes high water sensitivity to paint films. EP 0707 110 Al describes an acrylic emulsifier prepared by conventional latex polymerization from about 60% acrylic acid, 10% stearyl methacrylate, 30% butyl acrylate, and a chain transfer agent in water with a sulfate surfactant. The resulting emulsion, however, does not have uniform polymer composition and exhibits inferior properties due to being prepared by a normal latex process. The long chain alkyl monomer, stearyl methacrylate together with acrylic acid and butyl acrylate, is fed into a mixture of water and surfactant without pre-emulsification. The acrylic acid and butyl acrylate, being partially water soluble, migrate through the aqueous phase to the polymer particles leaving the water insoluble stearyl methacrylate behind in large monomer droplets. Polymer formed in the polymer particles is a copolymer of butyl acrylate and acrylic acid while stearyl methacrylate is left to homopolymerize or remain unreacted.
In contrast, the process and the polymers prepared in accordance with this invention provide useful polymeric thickeners of a uniform polymeric particle composition and free of organic solvents or diluents. Miniemulsion copolymers prepared by this invention provide excellent polymeric thickeners for various water dispersed products, especially in the formation of emulsion polymers for latex paints. By using a carboxyl monomer and long alkyl chain acrylate or methacrylate hydrophobic monomers, along with other monomers, the resulting miniemulsion polymeric thickeners minimize water sensitivity problems and increase thickening properties considerably to enable reduction in the amounts of thickener and paint solids required to maintain the desired paint viscosity. High opacity contrast ratios are maintained while shim scrub resistance is improved. The miniemulsion copolymer thickeners provide excellent viscosity build in latex paints and further provide a dispersant for pigments. These and other advantages of this invention will become more apparent by referring to the detailed description and the illustrative examples.